1. Field of the Invention
This invention relates to thermodynamically miscible, transparent single phase blends comprising: (I) an aromatic polysulfone (PSF); and (II) an aromatic alkyl methacrylate polymer (PAAM) such as polybenzyl methacrylate, polyphenylethyl methacrylate, or poly(2,6-dimethylphenyl)methacrylate.
2. Discussion of the Background
The term "thermodynamically miscible blend" defines a polymer mixture that mixes on the molecular level so as to form a single homogeneous phase which exhibits only one glass transition (Tg). This term is used in comparison to the term "mechanically compatible" which means that mixing of the polymers is on a small scale but larger than that of the molecules. Furthermore, mechanical compatibility implies that the multiple phases exhibit good adhesion to one another so as to yield good mechanical properties. Although both thermodynamically miscible and mechanically compatible blends exhibit good mechanical properties, only thermodynamically miscible systems are transparent owing to their single phase nature.
Aromatic polysulfones (PSF) typically are transparent, amorphous thermoplastics with a good balance of properties, most notably excellent electrical properties, creep resistance, high tensile strength, good thermal stability and resistance to many acids, bases and solvents. Nevertheless, they suffer from two major drawbacks. The first is that many potentially useful compositions are difficult to process. The second is that they have poor impact strength. Both of these drawbacks lead to relatively high production costs. This problem explains why only two such materials are commercially available, a polymer of 4-4'-chlorophenylsulfonylphenol, sold under the trade names of Victrex.RTM. and Ultrason.RTM. by ICI and BASF respectively, and a copolymer of 2,2-bis-4-hydroxyphenylpropane (Bisphenol A) with 4,4'-dichlorodiphenylsulfone, sold under the name Udel.RTM. by Amoco Performance Products.
Polyaromatic alkylmethacrylate polymers (PAAM) exhibit properties typical of most methacrylate polymers with the exception that they generally have higher heat resistance. Thus they exhibit good clarity, surface hardness, UV resistance, good weatherability and chemical resistance. For these reasons they can be used in applications where clarity and durability are essential. Although PAAM's have improved thermal stability over most alkyl methacrylate polymers, they are limited by their relatively poor dimensional stability compared to aromatic backbone-based polymers such as PSF.
Blends of polysulfones with other polymers could, in principle, eliminate the deficiencies of PSF and result in materials having considerably improved mechanical properties and ease of processability. Unfortunately PSFs are not thermodynamically miscible with most other polymers. Their blends are opaque materials and unacceptable for most applications requiring both transparency and mechanical performance. The two-phase nature of these blends does not improve the processability of the compositions either. Nonetheless, blend modification of polysulfone is known and practiced. Blends of polysulfones with polyvinylchloride (PVC), polystyrene, styrene-acrylonitrile copolymer (SAN), acrylonitrile-butadiene-styrene copolymer (ABS), styrene-methylmethacrylate copolymers, polyurethanes and nitrile rubbers have all been reported to be incompatible. (S. Kraus, Chapter 2 in Polymer Blends, Academic Press, 1978). For commercial purposes, polysulfone is often blended with ABS to improve the polysulfone's inherently poor impact strength but this method results in loss of transparency due to the incompatibility of the two materials. Thus, one cannot maintain the beneficial properties of PSF and its clarity by use of ABS.
Structural changes to the backbone of polysulfones, such as methyl substitution, have also been reported and can bring about miscibility with certain polymers, e.g., SAN (Olabasi, et al in Polymer-Polymer Miscibility, Academic Press, 1979). Unfortunately, because of the complexity of these chemical modifications, such materials are not commercially viable. Therefore the need existed for transparent and easily processable polymer blends containing aromatic polysulfones.
We have found that blends of aromatic alkyl methacrylate polymers with aromatic polysulfones have a high degree of compatibility over all composition ranges and can overcome the deficiencies of PSF. They provide single phase, transparent alloys with a range of mechanical and optical properties suitable for a number of commercially viable applications. Thermodynamically miscible blends of PSF and PAAM eliminate the individual deficiencies of the respective components while resulting in materials having considerably improved mechanical properties and ease of processing for a wide range of applications. Most importantly this improvement in properties occurs without loss of transparency so that even the most demanding applications requiring both mechanical performance and optical clarity can be achieved.